Laser lift off systems and methods

ABSTRACT

Laser lift off systems and methods may be used to provide monolithic laser lift off with minimal cracking by reducing the size of one or more beam spots in one or more dimensions to reduce plume pressure while maintaining sufficient energy to provide separation. By irradiating irradiation zones with various shapes and in various patterns, the laser lift off systems and methods use laser energy more efficiently, reduce cracking when separating layers, and improve productivity. Some laser lift off systems and methods described herein separate layers of material by irradiating non-contiguous irradiation zones with laser lift off zones (LOZs) that extend beyond the irradiation zones. Other laser lift off systems and methods described herein separate layers of material by shaping the irradiation zones and by controlling the overlap of the irradiation zones in a way that avoids uneven exposure of the workpiece. Consistent with at least one embodiment, a laser lift off system and method may be used to provide monolithic lift off of one or more epitaxial layers on a substrate of a semiconductor wafer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of co-pending U.S. patent applicationSer. No. 12/962,068 filed Dec. 7, 2010, which claims the benefit of U.S.Provisional Patent Application Ser. No. 61/267,194 filed Dec. 7, 2009,which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to separation of layers of material andmore particularly, to a laser lift off systems and methods formonolithic separation of layers of material, such as a substrate and afilm grown on the substrate.

BACKGROUND INFORMATION

Laser lift off may be used to separate layers of material. Oneapplication in which laser lift off has been used advantageously is theseparation of GaN layers from sapphire substrates when manufacturinglight emitting diodes (LEDs). In spite of the advantages from UV-laserlift-off, GaN LED manufacturing has been limited due to poorproductivity caused by low process yield. The low yield is due in partto high residual stresses in a GaN-sapphire wafer, resulting from aMetal-Organic Chemical Vapor Deposit (MOCVD) process. The MOCVD processrequires an activation temperature of over 600° C. As shown in FIG. 1A,GaN and InGaN layers 32 are deposited on a sapphire wafer 38 by theMOCVD process. Since there is substantial difference in coefficients ofthermal expansion (CTE) between the GaN (5.59×10−6/° K) and the sapphire(7.50×10−6/° K) (see Table 1), high levels of residual stresses existwhen the GaN/sapphire wafer cools down to ambient temperature from thehigh temperature of the MOCVD process, as shown in FIG. 1B. The residualstresses include compressive residual stresses 40 on the GaN andtensional residual stresses 42 on the sapphire.

TABLE 1 Various material properties of GaN and sapphire. Band ThermalLattice Lattice Den- Gap Expan- Const. a Const. c sity Energy sion ×Material Structure (Å) (Å) (g/cm³) (eV) 10⁻⁶/° K Sapphire Hexagonal4.758 12.991 3.97 9.9 7.50 GaN Hexagonal 3.189 5.815 6.1 3.3 5.59

When an incident laser pulse with sufficient energy hits a GaN/sapphireinterface, the irradiation results in instantaneous debonding of theinterface. Since the incident laser pulse has limited size (usually farless than 1 cm²), it creates only a small portion of the debonded orlifted-off interface. Since surroundings of the debonded area still havea high level of residual stress, it creates a concentration of stress atthe bonded/debonded border, resulting in fractures at the border. Thisfracturing, associated with the residual stress, has been one of theobstacles of the UV-laser lift-off process.

Currently, there are different ways to perform laser lift-off processeson GaN/sapphire wafers. One method involves raster scanning of aQ-switched 355 nm Nd:YAG laser (see, e.g., M. K. Kelly, R. P. Vaudo, V.M. Phanse, L. Gorgens, O. Ambacher and M. Stutzmann, Japanese Journal ofApplied Physics, vol. 38 p. L217, 1999). This lift-off process using asolid state laser is illustrated in FIG. 2A. Another method uses a 248nm excimer laser (see, e.g., W. S. Wong, T. Sands, N. W. Cheung, M.Kneissl, D. P. Bour, P. Mei, L. T. Romano and N. M. Johnson, AppliedPhysics Letters, vol. 75 p. 1360, 1999). This lift-off process using anexcimer laser is illustrated in FIG. 2B.

Both processes employ raster scanning, as shown in FIG. 3, whichinvolves either translation of the laser beam 44 or the target of theGaN/sapphire wafer 46. A problem associated with the raster scanningmethod is that it requires overlapping exposures to cover the desiredarea, resulting in multiple exposures 48 for certain locations. In bothof the above methods, the laser lift-off of GaN/sapphire is a singlepulse process. The unnecessary multiple exposures in localized areasincrease the potential for fracturing by inducing excessive stresses onthe film.

As shown in FIG. 4, raster scanning also involves a scanning of thelaser beam 44 from one end to the other, gradually separating theGaN/sapphire interface from one side to the other. This side-to-siderelaxation of residual stresses causes large differences in the stresslevel at the interface 50 between the separated and un-separatedregions, i.e., the interface between the scanned and the un-scannedarea. The disparity in residual stress levels at the interface 50increases the probability of propagation of Mode I and Mode II cracks.Although the illustrations in FIGS. 3 and 4 are based on a process usinga solid state laser, raster scanning of an excimer laser will producesimilar results.

Currently, a common size of sapphire wafers is two-inch diameter, butother sizes (e.g., three-inch and four-inch wafers) are also availablefor the hetero-epitaxial growth of GaN. For a GaN/sapphire wafer, thelevel of residual stresses varies in the wafer, and compressive andtensile residual stresses may exist together. The existence of theresidual stresses may be observed by wafer warping or bowing. When alaser lift-off process relaxes a large area of a continuous GaN/sapphireinterface, as described above, a severe strain gradient may be developedat the border between the debonded and the bonded interface. This straingradient may cause extensive fracturing of the GaN layer.

When a target material is irradiated with an intense laser pulse, ashallow layer of the target material may be instantaneously vaporizedinto the high temperature and high pressure surface plasma. Thisphenomenon is called ablation. The plasma created by the ablationsubsequently expands to surroundings. The expansion of the surfaceplasma may induce shock waves, which transfer impulses to the targetmaterial. The ablation may be confined in between two materials when thelaser is directed through a transparent material placed over the target.During this confined ablation, the plasma trapped at the interface maycreate a larger magnitude of shock waves, enhancing impact pressures.The explosive shock waves from the confined ablation at the GaN/sapphireinterface can cause not only separation of the GaN layer from thesapphire substrate but may also fracture the GaN layer near the laserbeam spot (see, e.g., P. Peyre et. al., Journal of Laser Applications,vol. 8 pp. 135-141, 1996).

Another technique for laser lift off involves the use of near-fieldimaging techniques to image a beam spot at the interface between thelayers being separated. FIG. 5 illustrates one example of a projectionof a homogeneous beam by near-field imaging and shows a representativebeam profile along the beam path. The raw beam from an excimer laser 120has Gaussian distribution in short side and flat topped distribution inthe long side. The beam homogenizer 122 (e.g., of multi-arrayconfiguration) makes the gradient raw beam profile into a squareflat-topped profile. The homogenized beam is cropped by the mask 124(e.g., the rectangular variable aperture) to utilize the best portion ofthe beam, which is projected to the LED target wafer 116 by near-fieldimaging, for example, using beam imaging lens 126. The edge resolutionof the homogeneous beam spot 130 at the LED wafer 116 therefore becomessharp.

FIGS. 6-8C illustrate one way in which the imaged beam (e.g., the beamspot 130) can cause separation of layers of material of the LED wafer116. Referring to FIG. 6, the laser may be directed through at least onelayer of substrate material 102 (e.g., sapphire) to at least one targetmaterial 104 (e.g., GaN) to separate the materials 102, 104. Theseparation of the materials 102, 104 may be achieved by using a laserenergy density sufficient to induce a shock wave at the interface 106 ofthe target material 104 and the substrate material 102, therebyinstantaneously debonding the target material 104 from the substratematerial 102. The shock wave may be created by the explosive expansionof plasma 108 at the interface as a result of the increased density ofthe ionized vapor sharply elevating the plasma temperature. The laserenergy density may be in a range sufficient to induce a force F_(a) onthe target material 104 that causes separation without fracturing. Theapplied force F_(a) may be represented as follows:

P _(p)(GPa)=C[I _(r)(GW/cm²)]^(1/2)

F _(a)(N)=P _(p)(GPa)A _(r)(cm²)

where P_(p) is the peak pressure induced by explosive shock waves, C isan efficiency and geometrical factor, I_(r) is the irradiance of theincident laser beam, F_(a) is the applied force and A_(r) is the areaunder irradiation.

When the plasma 108 is expanding, as shown in FIG. 7, the irradiationzone is acting as a bending arm pivoting at the edge of the irradiationzone. For example, the force (F_(r)) required for rupturing orfracturing may be viewed as a two-point bend test and may be representedas follows:

$F_{r} \propto {\frac{w\; d^{2}}{L}\sigma_{r}}$

where d is the thickness of the target material 104, w is the width ofthe applied force or width of the laser pulse, L is the length ofapplied arm or half length of the laser pulse, and σ_(r) is the modulusof rupture or fracture stress of GaN. To increase the force (F_(r)), thewidth w of the laser pulse may be increased and the half length L of thelaser pulse may be decreased, thereby forming a line shaped beam. Theline shaped beam may be scanned across the target material 104 tominimize the bending moment upon irradiation.

At a laser energy density under the ablation threshold of GaN (˜0.3J/cm² at 248 nm), for example, the instantaneous separation of theGaN/sapphire interface 106 may not be successfully achieved, as shown inFIG. 8A. Although decomposition of the GaN can occur under the ablationthreshold, this alone cannot achieve instantaneous separation of theinterface 106, because there is no driving force, i.e. shock waves fromthe expanding plasma, without the ablation. Conversely, applyingoverly-intense laser energy density may create excessive explosivestress wave propagation, which results in cracks and fractures on thetarget material 104 (e.g., the GaN film), as shown in FIG. 8C. When theirradiating laser energy density is optimized, as shown in FIG. 8B, theforce created by the shock wave is sufficient to separate the layers102, 104 at the interface 106 but not enough to induce fracture in thetarget material 104. According to this example with GaN and sapphire,the laser energy density may be between about 0.60 J/cm² to 1.5 J/cm² toachieve the separation shown in FIG. 8B.

The near-field imaging techniques described above have been usedsuccessfully in a process known as patterned laser lift off to overcomemany of the problems associated with residual stress and other problemsdiscussed above. The patterned laser lift off technique forms streets inone or more layers to be separated, forms a beam spot to cover one ormore of the sections defined by the streets, and separates layers in thesections. One example of a patterned lift off method is described ingreater detail in U.S. Pat. No. 7,202,141, which is fully incorporatedherein by reference. Although successful, the patterned lift offtechnique requires the additional steps of forming the streets, whichresults in a more time-consuming process. Attempts at using thetechniques described above to provide monolithic laser lift off,however, have been less successful because of the problems associatedwith residual stress.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will be better understood byreading the following detailed description, taken together with thedrawings wherein:

FIG. 1A is a schematic diagram illustrating a GaN/sapphire wafer duringa MOCVD process.

FIG. 1B is a schematic diagram illustrating formation of residualstresses on a GaN/sapphire wafer after a MOCVD process.

FIG. 2A is a schematic diagram illustrating a conventional method oflaser lift-off on a GaN/sapphire wafer using a Q-switched 355 nm Nd:YAGlaser.

FIG. 2B is a schematic diagram illustrating a conventional method oflaser lift-off on a GaN/sapphire wafer using a 248 nm excimer laser.

FIG. 3 is a schematic diagram illustrating raster scanning of aQ-switched 355 nm Nd:YAG laser on a GaN/sapphire LED wafer and theresulting multiple exposures.

FIG. 4 is a schematic diagram illustrating raster scanning on aGaN/sapphire LED wafer and the resulting stresses, which create a highprobability of Mode I and II cracks at the interface.

FIG. 5 is a schematic diagram of a laser lift off system that usesnear-field imaging to form a beam spot.

FIG. 6 is a schematic diagram of the use of a laser pulse to induce ashock wave for separating layers.

FIG. 7 is a schematic diagram illustrating an irradiation zone andcross-section of the separation of the layers.

FIGS. 8A-8C are schematic diagrams illustrating the effects of differentlaser energy densities on the separation of layers.

FIGS. 9A and 9B are schematic perspective and side views, respectively,of a reduced size beam spot irradiating layers of material with anirradiation zone formed at the interface and a lift off zone (LOZ)extending around the irradiation zone.

FIGS. 10A-10D show a reduced size beam spot being scanned with varyingdegrees of overlap, consistent with various embodiments.

FIGS. 10E and 10F are a schematic plan and side views, respectively, oflaser beams forming laser beam spots that irradiate non-contiguousirradiation zones.

FIG. 11 is a schematic perspective view of a laser lift off systemincluding a diffractive optical element (DOE) forming a pattern ofnon-contiguous laser beam spots capable of irradiating a correspondingpattern of simultaneous non-contiguous irradiation zones.

FIGS. 12A and 12B are schematic perspective views of a mask forming apattern of non-contiguous laser beam spots capable of irradiating acorresponding pattern of simultaneous non-contiguous irradiation zones,consistent with different embodiments.

FIG. 13 is a schematic perspective view of a laser lift off systemincluding a galvanometer capable of moving one or more laser beams toirradiate a pattern of non-contiguous irradiation zones.

FIG. 13A is a schematic perspective view of a galvanometer scanning aline shaped beam.

FIGS. 14A-14M are schematic illustrations of laser beam spots havingvarious shapes for irradiating irradiation zones with correspondingLOZs.

FIGS. 15A-15G are schematic illustrations of irradiation patterns formedby non-contiguous laser beam spots and lift off patterns formed byscanning the irradiation patterns for sequential irradiations withdifferent degrees of negative overlap.

FIGS. 16A-16E are schematic illustrations of irradiation patterns formedby non-contiguous laser beam spots and lift off patterns formed byscanning the irradiation patterns for sequential irradiations with zerooverlap and with positive overlap.

FIGS. 17A-17C are schematic illustrations of irradiation patterns formedby non-contiguous laser beam spots and lift off patterns formed byscanning the irradiation patterns for sequential irradiations withselected positive overlap.

FIGS. 18A-18B are schematic illustrations of irradiation matrix patternsformed by non-contiguous circular beam spots.

FIGS. 18C-18F are photomicrographs illustrating different irradiationpatterns formed by a matrix of beam spots having different shapes,sizes, spacings and fluence levels.

FIGS. 18G-18I are schematic illustrations of irradiation patterns formedby narrow line-shaped beam spots.

FIG. 18J is a schematic illustration of an irradiation pattern formed bymultiple matrices of small beam spots.

FIG. 18K is a schematic illustration of an irradiation pattern formed bymultiple hexagonal shaped line beam spots.

FIGS. 18L and 18M are schematic illustrations of a chevron shapedirradiation pattern formed by multiple angled line beam spots and anirradiation pattern formed by multiple chevron shaped irradiationpatterns.

FIG. 18N is a schematic illustration of an irradiation pattern formed bymultiple zig-zag shaped line beam spots.

FIG. 18O is a schematic illustration of an irradiation pattern formed bymultiple dashed shaped line beam spots.

FIGS. 19A-19D are schematic illustrations of irradiation patterns formedby non-contiguous laser beam spots scanned on a circular shapedworkpiece.

FIG. 20A is a schematic illustration of a beam spot moving in a spiralpattern to irradiate a circular shaped workpiece.

FIG. 20B is a schematic illustration of an annular beam spot moving toirradiate a circular shaped workpiece.

DETAILED DESCRIPTION

Laser lift off systems and methods, consistent with the embodimentsdescribed herein, may be used to provide monolithic laser lift off withminimal cracking by reducing the size of one or more beam spots in oneor more dimensions to reduce plume pressure while maintaining sufficientenergy to provide separation. By irradiating irradiation zones withvarious shapes and in various patterns, the laser lift off systems andmethods use laser energy more efficiently, reduce cracking whenseparating layers, and improve productivity. Some laser lift off systemsand methods described herein separate layers of material by irradiatingnon-contiguous irradiation zones with laser lift off zones (LOZs) thatextend beyond the irradiation zones. Other laser lift off systems andmethods described herein separate layers of material by shaping theirradiation zones and/or by controlling the overlap of the irradiationzones in a way that avoids uneven exposure of the workpiece. Consistentwith at least one embodiment, a laser lift off system and method may beused to provide monolithic lift off of one or more epitaxial layers on asubstrate of a semiconductor wafer.

According to some exemplary embodiments, the laser lift off systems andmethods described herein are used to lift off or separate a GaN epilayer from a sapphire substrate of a semiconductor wafer; although othertypes of substrates and layers of material may be used which are knownto those skilled in the art. Also, a sacrificial layer can be providedbetween the GaN (or other layer of material) and the sapphire (or othertype of substrate). Any of the lift off systems and methods describedherein can be applied to any highly absorbing material on a transparentcarrier including, without limitation, a polymer material such aspolyamide on transparent glass.

Referring to FIGS. 9A and 9B, a laser beam 1000 is used to separate oneor more layers 1002, 1004 of material of a workpiece 1001. The laserbeam 1000 passes through at least one layer 1002 to irradiate aninterface 1006 with at least one other layer 1004 to be separated orlifted off. As used herein, “irradiate” refers to exposing an area witha single pulse of laser radiation. A beam spot formed by the laser beam1000 at the interface 1006 irradiates an exposed laser irradiation zone1010 and causes separation of the layers 1002, 1004 in a lift off zone(LOZ) 1012, which may extend beyond the irradiation zone 1010. Therefractive index of the layer 1002 (e.g., the sapphire) through whichthe laser beam 1000 passes may also cause the laser light to bend inside(not shown) to create smaller irradiated beam spot on the layer 1004 ascompared to the beam 1000 or beam spot as seen on a surface of the layer1002. This smaller beam spot size on the layer 1004 may be taken intoconsideration when determining the spot size, spot spacing, and processparameters using the lift off methods described below to avoid crackingwhile allowing monolithic lift off.

The energy of a laser pulse causes ablation of the material in one ofthe layers 1002, 1004 primarily in the irradiation zone 1010 where thepulse energy is absorbed. This ablation results in an instantaneousdebonding and separation of the layers 1002, 1004. Because thermalenergy may be conducted beyond the irradiation zone 1010, ablationand/or decomposition may occur beyond the irradiation zone 1010 and mayresult in separation of the layers 1002, 1004 beyond the irradiationzone 1010 (i.e., creating the LOZ 1012). The ablation of material at theinterface 1006 also generates a plasma plume that expands and creates apressure (referred to as plume pressure) between the layers 1002, 1004,which may also extend the lift off zone 1012.

As mentioned above, when the plume pressure is too high, the resultingforce may cause cracking in one or more of the layers 1002, 1004 beingseparated. Consistent with the lift off systems and methods describedherein, laser irradiation parameters such as laser wavelength, pulsewidth, energy density, beam spot size, beam spot shape, and beam spotarray patterns may be configured to provide adequate separation of thelayers 1002, 1004 while minimizing plume pressure to minimize cracking.Consistent with at least some embodiments, the laser irradiationparameters may be configured to provide monolithic lift off of a GaNlayer on a substrate of a semiconductor wafer with minimal or nocracking.

The plume pressure is a function of the volume of material that isablated per pulse at the interface 1006 (also referred to as theinteraction volume), which relates to the area of the irradiation zone1010 and the ablation depth. Higher ablation rates, for example, mayremove a larger volume of material and create higher plume pressures.One way to reduce the interaction volume is by reducing the size of thelaser beam 1000 and thus the size of the beam spot. When reducing thespot size reduces the pulse energy applied to the irradiation zone 1010,the ablation depth may be reduced in addition to the area. As shown inFIG. 9B, for example, the laser beam 1000 is smaller than the laser beam10 and forms a beam spot (and irradiation zone) having a smaller areaand lower pulse energy. As a result of the smaller area of theirradiation zone and the lower pulse energy reducing the ablation depth,the volume of material 1011 ablated by a pulse of the smaller beam 1000is smaller than a volume of material 11 ablated by a pulse of the largerbeam 10.

Thus, the beam may be adjusted to reduce the beam spot size to match anallowable stress level for the materials 1002, 1004 being separated.Various sizes of beam spots may be used depending upon, for example, thetype of material, the thickness of the layers, and the area to beseparated. Examples of smaller beam spots that have been found capableof monolithic lift off of epi layers from sapphire substrates include100 by 100 micron square beams and 200 by 200 micron square beams. Asdescribed in greater detail below, the number of beam spots with reducedsize on target may be extended to achieve a higher throughput andvarious spacings may be used between irradiations (e.g., positive,negative or zero overlapping) to allow separation while minimizecracking. The spacings or overlapping may also be different in differentaxes or scan direction, for example, depending upon beam divergence inthe axes and/or the internal stresses along the axes. A beam passingthrough a homogenizer, for example, may have an asymmetrical pointspread function such that the beam profile is sharper along its Y axisthan along its X axis. Such a beam may have positive overlap along the Xaxis and zero or negative overlap along the Y axis.

Another way to change the interaction volume is by using differentwavelengths and/or pulse durations or widths. Depending upon thematerial characteristics, the ablation depth may be related to opticalpenetration depth and/or thermal penetration depth. Changing thewavelength may impact optical penetration depth and changing the pulsewidth may impact both optical penetration depth and thermal penetrationdepth. Shorter wavelengths having a higher photonic energy, for example,may provide better absorption and shallower optical penetration depthfor certain materials such as GaN. Shorter pulse widths reduce diffusioneffects because the pulse may be delivered in a shorter time than thetime it takes to conduct heat into the material and thus reduce thermalpenetration depth. Shorter pulse widths may also increase non lineareffects resulting in higher absorption and reduced optical penetrationin certain cases. Depending upon the materials, for example, ananosecond excimer laser at 248 nm or 193 nm or an ultrashort laser(with pulse durations less than 1 ns) may be used to provide a desiredablation depth.

A further way to change the interaction volume is by adjusting theenergy density of the beam spot that irradiates the irradiation zone1010. Ablation occurs when the energy density is above an ablationthreshold for a particular material and wavelength. As the energydensity increases, the absorption depth increases resulting in a largerinteraction volume. In general, energy density is energy over area andmay be controlled by changing either the energy or the area of the beam.For example, energy density may be controlled by controlling the powerto the laser, by using attenuators, or by shaping (e.g., expanding orcontracting) the beam using beam shaping optics, as will be described ingreater detail below. Thus, the energy density of the beam spot on theworkpiece may be varied to optimize the fluence and coupling efficiencyfor a particular material, to provide a desired ablation depth, and/orto control the amount of heat in the layer being lifted off.

The laser irradiation parameters, such as the wavelength, pulse width,and energy density, depend on the types of materials being separated.For example, a laser wavelength of 248 nm is suitable for separating GaNfrom sapphire because the photonic energy of 248 nm (5 eV) is betweenthe bandgaps of GaN (3.4 eV) and sapphire (9.9 eV). Thus, the 248 nmradiation is better absorbed in GaN than in sapphire and the selectiveabsorption allows the laser radiation to pass through the sapphire toablate the GaN resulting in separation. At a wavelength of 248 nm, theablation threshold of GaN is about 0.3 J/cm².

Those skilled in the art will recognize that other laser wavelengths maybe used to separate other types of materials. For example, a bufferlayer may be used between the sapphire substrate and the GaN layer(s) tofacilitate epitaxial growth of the GaN. Examples of the buffer layerinclude a GaN buffer layer and an Aluminum Nitride (AlN) buffer layer.Where an AlN buffer layer is used, a laser at 193 nm may be used becausethe photonic energy of the 193 nm laser light (6.4 eV) is in betweenbandgaps of sapphire (9.9 eV) and AlN (6.1 eV).

Referring to FIGS. 10A-10E, one or more reduced size beam spots may beused to irradiate multiple irradiation zones 1010 a-d with variousspacings between the irradiation zones to allow monolithic separation ofthe layers of the workpiece while minimizing cracking. FIGS. 10A-D showirradiation zones 1010 a-d formed by sequential irradiations using asingle beam spot with varying degrees of overlap as the beam spot movesstepwise across the workpiece. Four irradiation zones 1010 a-d are shownfor purposes of illustration, but multiple irradiation zones may besequentially formed across an entire workpiece to separate the layers ofthe workpiece. To move the beam spot stepwise across the workpiece, thebeam may be moved relative to the workpiece or the workpiece may bemoved relative to the beam, as will be described in greater detailbelow. The beam spot may also be moved in different directions orpatterns other than a stepwise direction across a workpiece, as will bedescribed in greater detail below.

As shown in FIG. 10A, for example, the irradiation zones 1010 a-d mayhave zero overlap (i.e., each step is substantially the same as thewidth of the beam spot). In other words, each location along theinterface between the layers is exposed to a single shot or pulse oflaser radiation. In one example where an epitaxial layer is separatedfrom a sapphire substrate, a single beam spot with a spot size of about227 microns by 213 microns at 1 J/cm² may be used with zero overlappingand when applied with a pulse frequency of about 400 Hz may be capableof lifting off a 2 inch wafer in about 3 minutes.

As shown in FIGS. 10B and 10C, the irradiation zones 1010 a-d may havepositive overlap such that each location along the interface between thelayers is exposed to multiple shots or pulses of laser irradiation. Inone embodiment, the positive overlap is controlled such that eachlocation is exposed to the same number of pulses to avoid unevenexposures across the workpiece. As shown in FIG. 10B, a square shapedbeam spot may be overlapped by using a step size about ½ the width ofthe beam spot such that each location is exposed to four (4) shots orpulses of laser irradiation. Beams spots with other shapes may also beused to control the overlap. As shown in FIG. 10C, for example, ahexagonal shaped beam spot may be overlapped by using a step size about½ the width of the beam spot such that each location is exposed to three(3) shots or pulses of laser irradiation. The energy density may be set(e.g., at about the ablation threshold) such that the multiple pulseswill result in separation without causing cracking.

In other embodiments, the irradiation zones may have a graduallydecreasing energy density at the edges and may overlap only at the edgessuch that the combined energy density of the overlapping portions isoptimized for separation without cracking. In these embodiments, agradient mask may be used to form the beam spots with the decreasingenergy density at the edges.

In other embodiments, a narrow line shaped beam may be scanned across aportion of a workpiece or across an entire workpiece with zero orrelatively small positive overlapping. Such a narrow line beam mayperform lift off without being homogeneous across the width of thenarrow line beam. In one example, a narrow line beam may have a lengthof about 52 mm and a width of about 20 microns and may be applied withan energy density of about 1 J/cm². Using such a narrow line beamgenerated by an excimer laser with a pulse frequency of 400 Hz may allowspeeds of 8 mm/sec and separation of an epi layer from a wafer in about10 sec. A line beam with a more narrow width may also be used.

As shown in FIG. 10D, the irradiation zones 1010 a-d may benon-contiguous or spaced with a negative overlap (i.e., the step size isgreater than the width of the beam spot). As used herein,“non-contiguous” refers to zones that do not overlap or touch. In thisembodiment, the beam spot may have a size and/or shape such that thelift off zone extends beyond the irradiation zone. Thus, the adjacentnon-contiguous irradiation zones 1010 a-d may be separated by a spacingthat is as wide as possible while still resulting in separation of thelayers. The pitch or separation may depend upon the type of materialbeing separated, the size of the lift off zone relative to theirradiation zone, and other laser processing parameters. In one examplewhere an epitaxial layer is separated from a sapphire substrate, asingle reduced size beam spot (e.g., about 200 by 200 microns) may besmaller than the step size or pitch by about 3 microns.

Although FIGS. 10A-10D show a single beam spot performing sequentialirradiations across a workpiece, an array or pattern of multiple reducedsize beam spots may also be formed to perform simultaneous irradiations.The reduced size beam spots within an array or pattern may have varyingdegrees of spacing or pitch. The array or pattern of beam spots may alsobe used to provide sequential irradiations with varying degrees ofoverlap similar to a single beam spot (i.e., zero overlap, positiveoverlap, or negative overlap). Although FIGS. 10A-10D show sequentialirradiations of either zero overlap, positive overlap, or negativeoverlap, various combinations of these overlaps may be used in differentaxes or scan directions.

As shown in FIGS. 10E and 10F, for example, multiple beams or beamlets1000 a-d may form an array of reduced size beam spots that irradiatemultiple non-contiguous irradiation zones 1010 a-d simultaneously. Inone embodiment, the array of beam spots may have a shape and size suchthat the lift off zones 1012 a-d extend beyond the respectiveirradiation zones 1010 a-d and the layers 1002, 1004 may be separatedwithout overlapping the irradiation zones 1010 a-d, thereby avoidingcracking due to multiple exposures. Where an array of non-contiguousirradiation zones 1010 a-d have larger lift off zones 1012 a-d, forexample, the aggregate lift off zone may be equivalent to one largerlaser beam spot 10.

As discussed above, the plume pressure is reduced by using the smallerbeam size and/or by otherwise reducing the interaction volume. Aplurality of non-contiguous irradiation zones 1010 a-d may be irradiatedwith a plume pressure that is less than the plume pressure of theequivalent larger beam spot 10 because the lift off zones 1012 a-d allowless energy to be spread over the same area with a smaller interactionvolume. Forming arrays of non-contiguous irradiation zones 1010 a-d maythus allow a larger aggregate lift off zone with less energy. The sizeand spacing of the beam spots may depend on the amount of energy densitythat is desired to provide separation with minimal pressure and stress.

Using a pattern of beam spots that simultaneously irradiate acorresponding pattern of non-contiguous irradiation zones 1010 a-dincreases the aggregate lift off zone of each irradiation and increasesthe lift off speed in addition to lowering the plume pressure and stresson the layers. Although FIGS. 10E and 10F shown non-contiguousirradiation zones with larger lift off zones, an array or pattern ofbeam spots may also irradiate non-contiguous irradiation zones with liftoff zones substantially the same as the irradiation zones. Variousshapes and patterns for beam spots and irradiation zones are describedin greater detail below.

The laser beams 1000 a-d and/or the workpiece 1001 may be moved suchthat the array of beam spots may perform irradiations in differentlocations around the workpiece 1001 to separate the layers 1002, 1004.The array or pattern of beam spots may be scanned or moved, for example,in a stepwise direction with varying degrees of overlapping irradiationssimilar to the single beam spot described above. Various scanningstrategies may be used across the workpiece 1001 to minimize thestresses that occur as a result of the separation processes (e.g.,leaving some rows or areas unprocessed and then filling in thoseunprocessed areas). Various patterns and scanning techniques are shownand described in greater detail below.

Embodiments of laser lift off systems are shown in FIGS. 11-13. Ingeneral, the laser lift off systems include a laser for generating a rawlaser beam and a beam delivery system for modifying the beam anddelivering the modified beam (or beamlets) to the workpiece 1001. Asshown in FIG. 11, a laser lift off system 1101 may include a laser 1120for generating a raw laser beam 1121, a beam shaper 1122 for producing ashaped laser beam 1123, and a diffractive optical element 1124 forforming multiple shaped beamlets 1100 a-d. The beam delivery system ofthe laser lift off system 1101 may also include one or more reflectiveelements 1126 for reflecting the laser beams.

The laser 1120 may include, without limitation, an excimer laser, adiode pumped solid state (DPSS) laser, a fiber laser, or an ultrafastlaser at the desired wavelength. An ultrafast laser is generally a lasercapable of emitting ultrashort pulses having pulse durations less than 1nanosecond, i.e., pulses with durations of femtoseconds or picoseconds.An ultrafast laser may be capable of producing the laser beam 1000 atdifferent wavelengths (e.g., about 0.35 μm, 0.5 μm or 1 μm or anyincrements therebetween) and at different ultrashort pulse widths (e.g.,less than about 10 ps). Using an ultrafast laser capable of changing thewavelength and/or pulse width allows control over the ablation depth andinteraction volume as described above. One example of an ultrafast laseris a Trumicro series 5000 picosecond laser available from TRUMPF.

The beam shaper 1122 may include, for example, a mask with an aperturethat produces the desired beam shape of the shaped laser beam 1123 whenthe raw laser beam 1121 passes through the aperture. The beam shaper1122 may also include beam shaping optics (e.g., lenses) that change theshape and/or size of the raw beam 1121 before a mask or the shaped beam1123 after a mask. In one embodiment, the beam shaper 1122 includes beamshaping optics capable of controlling the energy density of the beamspot, for example, as described in greater detail in U.S. Pat. No.7,388,172, which is fully incorporated herein by reference. The beamshaper 1122 may thus be used to vary the energy density of the beam spoton the workpiece (e.g., to reduce the absorption depth and theinteraction volume) without having to adjust the laser power. The beamshaper 1122 may further include beam shaping optics that produce thedesired beam shape of the shaped laser beam 1123 without using a mask.Using a beam shaper 1122 (e.g., a mask and/or beam shaping optics) tocontrol the shape and size of the beam spots and the energy density ofthe beam on the workpiece allows further control over the depth ofspreading of the energy and the geometrical spreading of the energy.

The diffractive optical element 1124 may include a holographic opticalelement (HOE) that uses the principles of diffraction to subdivide theshaped laser beam 1123 into the beamlets 1100 a-d. The beamlets 1100 a-dform an array of beam spots that simultaneously irradiate a pattern ofnon-contiguous irradiation zones 1010. The HOE uses substantially all ofthe energy of the shaped beam 1123 to form the beamlets 1100 a-d. Whenthe lift off zones exceed the irradiation zones of the beam spots, theenergy of the shaped beam 1123 may be spread over a larger area moreefficiently when using the HOE to form a pattern of smaller beamlets, ascompared to using a mask to image a single large beam spot. Using theHOE to form an array of smaller beam spots may also avoid the need tohomogenize the laser beam with a homogenizer. In one example, the HOEmay generate a spot array from a laser beam with an aggregate lift offzone that is greater than 4 times the lift off zone of a single beamspot formed by the same laser beam. One embodiment of a holographicoptical element also allows control over the beam spot size and thespacing (or pitch) of the array or pattern of beam spots formed by theHOE.

As shown in FIGS. 12A and 12B, a mask 1128 with an array of aperturesmay also be used to form an array or pattern of beam spots. One or morelaser beams may illuminate the mask 1128 to form the beamlets 1100 a-dthat pass through the apertures of the mask 1128. The shape, size andspacing of the apertures on the mask 1128 determines the shape, size andspacing of the beam spots produced by the beamlets 1100 a-d on thetarget. In one embodiment shown in FIG. 12A, a single homogenized beam1123 may be used to illuminate the mask 1128. In another embodimentshown in FIG. 12B, multiple beamlets 1123 a-d may be used to illuminatethe mask 1128 such that the beamlets 1123 a-d provide optimumillumination to match the apertures of the mask 1128, thereby reducingthe amount of beam energy that is rejected by the mask 1128 andincreasing the beam utilization factor. Other types of spot arraygenerators may also be used to form an array of beam spots.

The laser lift off system 1101 may also include a motion stage 1130 thatsupports the workpiece 1001 and positions the workpiece 1001 for thesequential irradiations. The motion stage 1130 may be an X-Y and/ortheta positioning stage capable of moving the workpiece 1001 in an X-Ydirection and/or rotating the workpiece 1001. A motion control system(not shown) may be coupled to the laser 1120 and motion stage 1130 tocontrol the laser irradiations and the positioning of the workpiece1001, for example, using a “fire on the fly” technique.

The laser lift off system 1101 may also control the manner in which thelayers are allowed to detach to minimize cracking. When the substrate1002 detaches as the lift off progresses across the workpiece 1001,stress is caused because part of the workpiece 1001 is being lifted offwhile part of the workpiece 1001 is not, which enhances cracking at thelift off front. The laser lift off system 1101 may thus include one ormore workpiece holders 1136 (e.g., clamps) for mechanically holding thesubstrate 1002 down as the lift off process takes place. The edges ofthe workpiece 1001 may be exposed first, for example, and then theworkpiece holders 1136 may hold the edges down while processing theinside region of the workpiece 1001. The workpiece holders 1136 may thenslowly release the substrate 1002. Alternatively, another transparentmaterial, such as a thin piece of sapphire, may be used over theworkpiece 1001, which allows the laser to pass through to perform thelift off and mechanically prevents bowing as a result of the lift off.By forcing the substrate, the global stresses and related crackingcaused by the lift off process may be minimized, thereby improving theyield when manufacturing LEDs.

According to another embodiment, shown in FIG. 13, a laser machiningsystem 1301 may include a galvanometer 1326 for scanning one or morebeam spots on the workpiece 1001 to provide a pattern of non-contiguousirradiation zones 1010. The galvanometer 1326 may be used instead of orin addition to moving the workpiece 1001 with a motion stage. Thegalvanometer 1326 may be a 1-D or 2-D galvanometer known to thoseskilled in the art for scanning a laser beam. Using the galvanometer1326 to scan the beam spot(s) increases the speed at which the beamspot(s) may be moved across the workpiece 1001 and thus increases thelift off speeds. In this embodiment of the laser lift off system 1301, abeam shaper 1322 (e.g., a mask and/or beam shaping optics) may be usedbefore the galvanometer 1326 to provide the desired shape of the beamspot(s) to be scanned across the workpiece 1001 and/or to control theenergy density of the beam spot(s). FIG. 13A shows another embodiment inwhich a galvanometer 1326 scans a line shaped beam.

FIGS. 14A-14M show various possible shapes for the beam spots andirradiation zones 1010. Depending upon the size and energy density ofthe beam spots and the materials being separated, each of theirradiation zones 1010 may have corresponding lift off zones (LOZs) 1012shown in broken lines. The size and energy density of the beam spots mayalso be adjusted such that the LOZ 1012 is essentially the same as theirradiation zone 1010. The possible shapes include, but are not limitedto, a square (FIG. 14A), circle (FIG. 14B), rectangular line (FIG. 14C),triangle (FIG. 14D), cross (FIG. 14E), L shape (FIG. 14F), hollow square(FIG. 14G), hollow circle (FIG. 14H), hexagon (FIG. 14I), ellipticalline (FIG. 14J), and arc (FIG. 14K). For applications where a maximizedlift off zone is desired, any shape may be used that provides a lift offzone that is maximized or relatively large compared to the irradiationzone. The beam spots may be formed with the various shapes using a maskand/or beam shaping optics.

Beam spots may also be formed with combinations of complimentary shapes,such as an L shape together with a square (FIG. 14L) and two L shapes(FIG. 14M). Various combinations of shapes may be formed together, forexample, to maximize the aggregate lift off zone formed relative to thecorresponding irradiation zones.

Multiple non-contiguous beam spots may be irradiated simultaneously on aworkpiece such that the corresponding irradiation zones form anirradiation pattern. The irradiation pattern may be scanned orpositioned at various locations across a workpiece by sequentiallyirradiating the irradiation zones in the irradiation pattern, therebyforming a lift off pattern including multiple sequential irradiationpatterns that achieve lift off across the workpiece. Each sequentialirradiation may involve a single pulse of laser energy with theirradiation pattern at a different location on the workpiece. Thesequential irradiations may have varying degrees of overlap (e.g., zerooverlap, positive overlap or negative overlap), which may be differentin different axes or scan directions.

FIGS. 15A-15G show various irradiation patterns that may be formed by anarray or pattern of non-contiguous beam spots and the various lift offpatterns that may be formed by scanning or positioning the irradiationpatterns across a workpiece with varying degrees of negative overlap ofthe irradiation zones. In the irradiation patterns shown in FIGS.15A-15G, the concept of a lift off zone extending beyond the irradiationzone allows the irradiation patterns to be scanned with negative overlapof the irradiation zones. Although the irradiation patterns are shownwith corresponding lift off zones (in broken lines) extending beyond theirradiation zones, the lift off zones may have a different size thanshown.

FIG. 15A shows an irradiation pattern 1500 a formed by a column ofsquare-shaped non-contiguous beam spots that are spaced such thatrespective LOZs are touching or overlapping. The aggregate lift off zoneformed by the irradiation pattern 1500 a thus forms a line. Theirradiation pattern 1500 a may be scanned across a workpiece withsequential irradiations to form different lift off patterns. Theirradiation pattern 1500 a may be scanned to provide a lift off pattern1502 a with the lift off zones of the sequential irradiations touchingor having minimal overlap. The irradiation pattern 1500 a may also bescanned to provide a lift off pattern 1504 a with the lift off zones ofthe sequential irradiations overlapping. The irradiation pattern 1500 amay also be scanned to provide a lift off pattern 1506 a with theirradiation zones of the sequential irradiations being contiguous.

FIG. 15B shows an irradiation pattern 1500 b formed by a column ofrectangular-shaped non-contiguous beam spots, which may be scannedacross the workpiece similar to the irradiation pattern 1500 a. Thisirradiation pattern 1500 b may be equivalent to a narrow line beamscanned across a workpiece.

FIG. 15C shows an irradiation pattern 1500 c formed by a multiplecolumns of square-shaped non-contiguous beam spots that are spaced suchthat respective lift off zones are touching or overlapping. Theirradiation pattern 1500 c may also be scanned to provide lift offpatterns 1502 c, 1504 c, 1506 c with varying degrees of overlap of thelift off zones of sequential irradiations. Although two columns areshown, the irradiation pattern 1500 c may also include more than twocolumns of square-shaped non-contiguous beam spots.

FIG. 15D shows an irradiation pattern 1500 d formed by a multiplestaggered columns of square-shaped non-contiguous beam spots that arespaced such that respective lift off zones are touching or overlapping.The irradiation pattern 1500 d may also be scanned to provide lift offpatterns 1502 d, 1504 d, 1506 d with varying degrees of overlap of thelift off zones of sequential irradiations.

FIG. 15E shows an irradiation pattern 1500 e formed by square-shapednon-contiguous beam spots arranged in a triangular pattern and spacedsuch that respective lift off zones are touching or overlapping. Theirradiation pattern 1500 e may also be scanned to provide lift offpatterns with varying degrees of overlap of the lift off zones ofsequential irradiations, such as the lift off pattern 1502 e.

FIG. 15F shows an irradiation pattern 1500 f formed by a square-shapednon-contiguous beam spots arranged in a diamond pattern and spaced suchthat respective LOZs are touching or overlapping. The irradiationpattern 1500 f may also be scanned to provide lift off patterns withvarying degrees of overlap of the lift off zones of sequentialirradiations, such as the lift off pattern 1502 f. The triangular shapedirradiation pattern 1500 e and the diamond shaped irradiation pattern1500 f may reduce point stresses when they are scanned to separate thelayers.

Instead of linearly scanning the irradiation patterns stepwise in onedirection across the workpiece, the irradiation patterns may also bepositioned in different locations across the workpiece. FIG. 15G showsan irradiation pattern 1500 g formed by square-shaped non-contiguousbeam spots arranged in an open square pattern. In this irradiationpattern 1500 g, the beam spots are spaced such that the lift off zonesaround the irradiation zones are non-contiguous. This irradiationpattern is formed at different locations to interdigitate the sequentialirradiations. As shown, the beam spots in this irradiation pattern 1500g are spaced such that multiple passes (i.e., sequential irradiations)are required to fill in an array of irradiation zones. For example, asecond pass of the irradiation pattern 1500 g forms the pattern 1502 g,a third pass of the irradiation pattern 1500 g forms the pattern 1504 g,and a fourth pass of the irradiation pattern 1500 g forms the completelift off pattern 1506 g that results in separation of the layers. Usingthis type of interdigitated scanning technique with the irradiationpattern 1500 g providing on/off areas of irradiation, the separation ofthe layers may be performed more gradually across an area of theworkpiece, thereby minimizing the stresses caused by releasing the layerfrom the substrate. In one example, the irradiation pattern 1500 g mayinclude an array of 100 micron square beam spots with a center to centerspacing of 500 microns.

FIGS. 16A and 16B show irradiation patterns that are formed by an arrayof beam spots that are spaced such that the irradiation patterns may bescanned or overlayed with zero overlap between the irradiation zones. Inthese irradiation patterns, the lift off zone may not extend beyond theirradiation zones. FIG. 16A shows an irradiation pattern 1600 a formedby two square-shaped non-contiguous beam spots that are spaced by aboutthe width of the beam spots (e.g., 200 by 200 microns spaced by 200microns). The irradiation pattern 1600 a may be scanned stepwise witheach step being substantially the same as the width of the square beamspot such that the irradiation zones are contiguous in the stepwisedirection. A single pass 1601 a of the irradiation pattern 1600 a formstwo rows of sequential irradiation zones that are spaced by a row thatis not irradiated or processed. The irradiation pattern 1600 a may bescanned with another pass to irradiate or “fill in” the unprocessed rowand form a complete lift off pattern 1602 a of irradiation zones thatcauses separation of the layers. Although the irradiation pattern 1600 ashown in FIG. 16A is formed by two square-shaped beam spots with aspacing equivalent to one beam spot, similar irradiation patterns may beformed by more than two beam spots with a spacing of one beam spot or aspacing of some multiple of the beam spot.

FIG. 16B shows an irradiation pattern 1600 b formed by square-shapednon-contiguous beam spots arranged in a staggered or “zig-zag” patternwith a spacing equivalent to about the width of the beam spots. Theirradiation pattern 1600 b may be scanned stepwise with each step beingsubstantially the same as the width of the square beam spot such thatthe irradiation zones are contiguous in the stepwise direction, therebyforming a lift off pattern 1602 b of irradiation zones that causesseparation of the layers. Although the irradiation pattern 1600 b shownin FIG. 16B is formed by four square-shaped beam spots with a spacingequivalent to one beam spot, similar irradiation patterns may be formedby more than four beam spots with a spacing of one beam spot or aspacing of some multiple of the beam spot. FIG. 16B-1 showsphotomicrographs of a wafer irradiated with the “zig-zag” irradiationpattern 1600 b.

FIG. 16C shows an irradiation pattern 1600 c formed by square-shapednon-contiguous beam spots arranged in a triangular pattern. Theirradiation pattern 1600 c may be scanned stepwise with each step beingsubstantially the same as the width of the square beam spot such thatthe irradiation zones are contiguous in the stepwise direction, therebyforming a lift off pattern 1602 c of irradiation zones that causesseparation of the layers.

FIG. 16D shows an irradiation pattern 1600 d formed by a square-shapednon-contiguous beam spots arranged in a square matrix pattern. Similarto the irradiation pattern 1500 g, this irradiation pattern is formed atdifferent locations to interdigitate the sequential irradiations. Asshown, the beam spots in this irradiation pattern 1600 d are spaced suchthat multiple passes (i.e., sequential irradiations) are required tofill in an array of irradiation zones. For example, a second pass of theirradiation pattern 1600 d forms the pattern 1602 d, a third pass of theirradiation pattern 1600 d forms the pattern 1604 d, and a fourth passof the irradiation pattern 1600 d forms the complete lift off pattern1606 d that results in separation of the layers. Other numbers andspacings of the beam spots may also be used to form a matrix pattern.

FIG. 16E shows an irradiation pattern 1600 e formed by ahexagonal-shaped non-contiguous beam spots arranged in a square matrixpattern. This irradiation pattern is formed at different locations tointerdigitate the sequential irradiations such that each of thehexagonal-shaped beam spots overlap similar to the hexagonal beam spotshown in FIG. 10C. As shown, the beam spots in this irradiation pattern1600 e are spaced such that multiple passes (i.e., sequentialirradiations) are required to fill in an array of irradiation zones withthe desired overlap. For example, a second pass of the irradiationpattern 1600 e forms the pattern 1602 e, a third pass of the irradiationpattern 1600 e forms the pattern 1604 e, and nine passes of theirradiation pattern 1600 e forms the complete lift off pattern 1606 ewith three overlapping exposures in each location, resulting inseparation of the layers. Other numbers and spacings of the hexagonalbeam spots may also be used to form a matrix pattern. An irradiationpattern may also be formed with hexagonal-shaped beam spots being spacedsuch that lift off zones overlap without overlapping the irradiationzones.

The irradiation patterns may also be scanned or positioned such that theirradiation zones in sequential irradiations overlap in selected areasof the lift off patterns, which may affect the amount of ablation thatoccurs in those areas. The irradiation pattern and the scanning may beconfigured to control the overlap and thus control the amount ofablation in the overlap areas, thereby providing various texturing orroughening effects on at least one of the layers (e.g., the GaN filmbeing lifted off). FIGS. 17A-17C show irradiation patterns formed bynon-contiguous beam spots, which may be scanned to form lift offpatterns with some overlap of the edges of the irradiation zones insequential irradiations to provide texturing or roughening. FIG. 17Ashows an irradiation pattern 1700 a that may be scanned to form a liftoff pattern 1702 a with intermittent overlapping irradiation zones. FIG.17B shows an irradiation pattern 1700 b that may be scanned to form alift off pattern 1702 b with overlapping irradiation zones formed alonglines in the scanning direction. FIG. 17C shows an irradiation pattern1700 c that may be scanned to form a lift off pattern 1702 c withintermittent overlapping irradiation zones extending in a directionorthogonal to the scanning direction. In the illustrated examples, thenon-contiguous beam spots forming the irradiation patterns 1700 a-c arerelatively narrow in the scanning direction and relatively wide in thedirection orthogonal to the scanning direction.

Although the irradiation patterns discussed above are shown with square,rectangular and hexagonal beam spots, these patterns may be formed usingbeam spots of any shape, number and spacing. FIGS. 18A and 18B, forexample, show irradiation patterns 1800 a, 1800 b formed by high densityarrays of circular beam spots. The irradiation patterns 1800 a, 1800 bmay be formed, for example, by passing a laser through a high densitymask such as an on-off mask with 10 micron diameter holes on 20 microncenter to center pitch.

FIGS. 18C-18F show photomicrographs of irradiation patterns formed atthe interface of a sapphire substrate and epi layer by arrays of beamspots with different pitches, different shapes and different fluencelevels. The lift off zone may be seen in these photomicrographs aroundthe irradiation zones. FIG. 18C shows photomicrographs of irradiationpatterns formed by arrays of circular beam spots having different sizesand pitches and with a high fluence. FIG. 18D shows photomicrographs ofirradiation patterns formed by arrays of circular beam spots havingdifferent sizes and pitches and with a low fluence. FIG. 18E showsphotomicrographs of irradiation patterns formed by arrays of square beamspots having different sizes and pitches and with a high fluence. FIG.18F shows photomicrographs of irradiation patterns formed by arrays ofsquare beam spots having different sizes and pitches and with a lowfluence. In these examples, the low fluence is just above the ablationthreshold.

FIG. 18G shows a square-shaped irradiation pattern 1800 g formed bymultiple narrow line-shaped beam spots. In one example, line-shaped beamspots with a length of about 300 microns and width of about 3 micronsmay be spaced about 10 microns to form a square-shaped irradiationpattern of about 300 microns by 300 microns. The square-shapedirradiation pattern 1800 g may be used in any of the irradiationpatterns discussed above, for example, instead of a solid square-shapedbeam spot and may be scanned or positioned across a workpiece using anyof the techniques described above to achieve lift off. As shown in FIG.18H, for example, multiple square-shaped irradiation patterns 1800 gmade up of narrow line-shaped beam spots may be arranged to form alarger irradiation pattern 1800 h.

Multiple line-shaped beam spots of different sizes may also be arrangedto form irradiation patterns of different sizes, shapes and/orgeometries. As shown in FIG. 18I, for example, multiple narrowline-shaped beam spots may be arranged end-to-end to form a longerline-shaped irradiation pattern 1800 i. In one example, line-shaped beamspots may be spaced only a few microns apart. The longer line-shapedirradiation pattern 1800 i may be scanned across a workpiece, forexample, instead of scanning a homogeneous line-shaped beam spot.

Instead of forming a square-shaped irradiation pattern with multiplenarrow line-shaped beam spots as shown in FIG. 18G, small on/off beamspots may be used to form a larger irradiation pattern (e.g., a squareshaped pattern of about 300 microns by 300 microns). FIG. 18J shows anirradiation pattern 1800 f formed by multiple square-shaped irradiationpatterns made up of small beam spots.

FIG. 18K shows a hexagonal shaped irradiation pattern 1800 k formed bymultiple nested hexagon shaped lines beams (i.e., instead of a solidhexagonal beam spot). The hexagonal shaped irradiation pattern 1800 kmay be arranged with other hexagonal shaped irradiation patterns 1800 kin a larger irradiation pattern. The hexagonal shaped irradiationpattern 1800 k may also be scanned or positioned with varying degrees ofoverlap as described above with a solid hexagonal beam spot.

FIG. 18L shows a chevron shaped irradiation pattern 18001 formed bymultiple angled line beams. The chevron shaped irradiation pattern 18001may be scanned with varying degrees of overlap, for example, asdescribed above with other patterns. Multiple chevron shaped irradiationpatterns 18001 may also be formed in a larger irradiation pattern. FIG.18M shows an irradiation pattern 1800 m formed by multiplechevron-shaped irradiation patterns 18001.

FIG. 18N shows an irradiation pattern 1800 n formed by multiple zig-zagshaped line beams and FIG. 18O shows an irradiation patterns 1800 oformed by multiple dashed line beams. These irradiation patterns 1800 n,1800 o may also be scanned with varying degrees of overlap and/or may beformed in a larger irradiation pattern.

By forming irradiation patterns from multiple lines (e.g., as opposed tosolid beam spots), the irradiation patterns may be overlapped withsequential radiation patterns with reduced cracking and ghostingeffects. The multiple lines provide substantially uniform irradiationand allow the edges of the irradiation pattern to be blended withoverlapping irradiation patterns. The irradiation patterns formed bymultiple lines may also be overlapped to achieve a roughening effect onthe layer being separated, for example, to roughen GaN for enhancing theLED performance. According to another variation, a solid beam spot maybe formed in any of the shapes described herein with multiple lines atone or more edges of the solid beam spots to allow overlap with minimalor no ghosting. According to a further variation, a beam spot may besolid at the center with feathered edges.

The irradiation patterns including a plurality of lines or a solid beamspot with lines at the edges, as described above, may be formed by aphotomask etched with the desired line pattern. One type of mask thatmay be used is a chrome/quartz on/off mask. The irradiation patternsincluding a plurality of lines may also be formed by shaping a beam froma high speed solid state laser to form the line pattern. A beam spotwith feathered edges may be formed by a gray scale mask.

FIGS. 19A-19D show irradiation patterns (with an aggregate LOZ shown inbroken lines) that are designed to irradiate a circular workpiece 1001such as a semiconductor wafer. In FIG. 19A, an irradiation pattern 1600a is formed by non-contiguous beam spots arranged in a pie-shapedpattern extending from the center to the circumference of the circularworkpiece 1001. The irradiation pattern 1600 a and/or the workpiece 1001may be rotated to scan the irradiation pattern 1600 a around theworkpiece. Although the pie-shaped irradiation pattern 1600 a is shownas a one-quarter circle, larger or smaller pie-shaped patterns may beused.

In FIG. 19B, an irradiation pattern 1600 b is formed by non-contiguousbeam spots arranged in a spoke pattern extending from the center to thecircumference of the circular workpiece 1001. The irradiation pattern1600 b and/or the workpiece 1001 may be rotated to scan the irradiationpattern 1600 a around the workpiece. Although the spoke irradiationpattern 1600 b is shown with four (4) spokes, other numbers of spokesmay also be used.

In FIG. 19C, an irradiation pattern 1600 c is formed by non-contiguousbeam spots arranged in an annular pattern around the workpiece 1001. Theannular irradiation pattern 1600 c may be scanned from the circumferenceto the center of the workpiece 1001 by moving the beam spots inwardly.

In FIG. 19D, an irradiation pattern 1600 d is formed by non-contiguousbeam spots arranged in a semi-annular or arc pattern around theworkpiece 1001. The semi-annular irradiation pattern 1600 d may bescanned around the circumference workpiece 1001 and may be scannedinwardly from the circumference to the center of the workpiece 1001.

The size, shape and number of beam spots used to form the irradiationpatterns 1600 a-d may be varied depending upon the size of theworkpiece, the materials and the desired energy density.

FIGS. 20A and 20B show other techniques for scanning beam spots toperform laser lift off on a circular shaped workpiece 1001. In FIG. 20A,a beam spot 1710 a (e.g., having a cross shape) may be scanned aroundthe workpiece 1001 in a spiral pattern. In one embodiment, the beam spot1710 a may be moved in the spiral pattern, for example, using agalvanometer. In another embodiment, the beam spot 1710 a may be movedradially toward the center while the workpiece 1001 is rotated. Althougha cross-shaped beam spot is shown, a beam spot or array of beam spotswith other shapes may also be scanned in a spiral direction.

In FIG. 20B, an annular beam spot 1710 b may be scanned inwardly with adecreasing diameter and width as the beam spot moves. The annular beamspot 1710 b may be formed, for example, using an acoustic lens. Theannular beam spot 1710 b may also be formed as a defocused annular beamthat maintains energy density while the width and arc length changes asthe beam is scanned inwardly. In another variation, a variable diameternarrow ring of laser light may be scanned from the outside to thecenter, for example, using an electro optic effect.

Consistent with a further embodiment, a laser lift off method includes:providing a workpiece including at least first and second layers ofmaterial; generating at least one laser beam; and irradiatingoverlapping irradiation zones at an interface between the first andsecond layers with a beam spot formed by the at least one laser beam andmoved in a stepwise direction, wherein the irradiation zones areoverlapped such that a substantial number of the locations at theinterface are exposed to the same number of pulses of laser irradiation,wherein the number of pulses of laser irradiation at the locations issufficient to cause separation of the layers.

Consistent with yet another embodiment, a laser lift off systemincludes: a laser for generating a raw laser beam; a beam shaper forshaping the raw laser beam into a shaped beam; and a spot arraygenerator for receiving the shaped beam and generating an array ofnon-contiguous beam spots arranged in a predefined pattern.

Consistent with yet another embodiment, a laser lift off systemincludes: a laser for generating a raw laser beam; a beam shaper forshaping the raw laser beam into a shaped beam and for changing theenergy density of the beam; and a galvanometer for scanning the shapedbeam spot on the workpiece.

While the principles of the invention have been described herein, it isto be understood by those skilled in the art that this description ismade only by way of example and not as a limitation as to the scope ofthe invention. Other embodiments are contemplated within the scope ofthe present invention in addition to the exemplary embodiments shown anddescribed herein. Modifications and substitutions by one of ordinaryskill in the art are considered to be within the scope of the presentinvention, which is not to be limited except by the following claims.

What is claimed is:
 1. A laser lift off method for separating layers ofmaterial, the method comprising: providing a workpiece including atleast first and second layers of material; generating at least one laserbeam; and irradiating overlapping irradiation zones at an interfacebetween the first and second layers with a beam spot formed by the atleast one laser beam and moved in a stepwise direction, wherein theirradiation zones are overlapped such that a substantial number of thelocations at the interface are exposed to the same number of pulses oflaser irradiation, wherein the number of pulses of laser irradiation atthe locations is sufficient to cause separation of the layers.
 2. Thelaser lift off method of claim 1 wherein the beam spot is moved with astep size about ½ a width of the beam spot.
 3. The laser lift off methodof claim 1 wherein the beam spot and irradiation zones are squareshaped, and wherein each location at the interface is exposed to fourpulses of laser irradiation.
 4. The laser lift off method of claim 1wherein the beam spot and irradiation zones are hexagonal shaped, andwherein each location at the interface is exposed to three pulses oflaser irradiation.
 5. The laser lift off method of claim 1 wherein thebeam spot and irradiation zones are rectangular shaped, and wherein theirradiation zones have different overlap in different directions.
 6. Alaser lift off system comprising: a laser for generating a raw laserbeam; a beam shaper for shaping the raw laser beam into a shaped beam;and a spot array generator for receiving the shaped beam and generatingan array of non-contiguous beam spots arranged in a predefined pattern.7. The laser lift off system of claim 6 wherein the laser includes anultrafast laser for generating an ultrashort pulse laser beam.
 8. Thelaser lift off system of claim 6 wherein the beam shaper includes amask.
 9. The laser lift off system of claim 8 wherein the beam shaperincludes beam shaping optics for shaping the beam and changing theenergy density of the beam.
 10. The laser lift off system of claim 8wherein the spot array generator includes a holographic optical element.11. The laser lift off system of claim 6 wherein the spot arraygenerator includes a mask including a plurality of apertures.
 12. Alaser lift off system comprising: a laser for generating a raw laserbeam; a beam shaper for shaping the raw laser beam into a shaped beamand for changing the energy density of the beam; and a galvanometer forscanning the shaped beam spot on the workpiece.
 13. The laser lift offsystem of claim 12 wherein the laser includes an ultrafast laser forgenerating an ultrashort pulse laser beam.
 14. The laser lift off systemof claim 12 wherein the beam shaper includes beam shaping optics. 15.The laser lift off system of claim 12 wherein the beam shaper includes amask.
 16. The laser lift off system of claim 12 wherein the beam shaperincludes beam shaping optics and a mask.